FOREWORD

This report provides direct input into the Accident Analysis Module (AAM) of
the Interactive Highway Safety Design Model. The AAM is a tool that highway
engineers can use to evaluate the impacts of highway design elements in project
planning and preliminary design. Three crash models were developed relating
crashes to three types of rural intersections. These types are: (1) three-legged
intersections with major four-lane roads and minor two-lane roads that are stop-controlled,
(2) four-legged intersections with major four-lane roads and minor two-lane
roads that are stop-controlled, and (3) signalized intersections with both major
and minor two-lane roads.

Elaborate sets of data were acquired from State data sources (Michigan and
California) and collected in the field. The final data sets consist of 84 sites
of three-legged intersections, 72 sites of the four-legged intersections, and
49 sites of the signalized intersections. Negative binomial models -- variants
of Poisson models that allow for overdispersion -- were developed for each of
the three data sets. Significant variables included major and minor road traffic;
peak major and minor left-turn percentage; peak truck percentage; number of
driveways; and channelization, intersection median widths, vertical alignment,
and, in the case of signalized intersections, the presence or absence of protected
left-turn phases. Separate models were developed for crashes resulting in injuries
and total crashes.

Michael F. Trentacoste
Director, Office of Safety
Research and Development

This report describes the collection, analysis, and modeling of crash and roadway data for intersections on rural roads in California and Michigan for
the years 1993–1995. Three classes of intersection are considered: (1) three–legged intersections with major four–lane roads and minor
two–lane roads that are stop–controlled, (2) four–legged intersections with major four–lane roads and minor two–lane roads
that are stop–controlled, and (3) signalized intersections with both major and minor two–lane roads. Data was acquired from the Highway Safety
Information System, State and Federal photologs, and field work at all intersections. The field work included morning and evening traffic counts by
movement and vehicle type as well as alignment measurements out to 800 feet (244 meters) along the major road. The final data sets consist of 84 three
–legged intersections, 72 four–legged intersections, and 49 signalized intersections. Other variables collected include Roadside Hazard Rating,
number of driveways, channelization, intersection angles, and speed limits.

Negative binomial models –– variants of Poisson models that allow for overdispersion –– were developed for each of the three data
sets. Significant variables included major and minor road traffic, peak major and minor left–turn percentage, number of driveways, channelization,
median widths, vertical alignment, and, in the case of signalized intersections, the presence or absence of protected left–turn phases and peak truck
percentage. Models are developed for all crashes within 250 feet (76 meters) of the intersection center, for intersection–related crashes within 250
feet (76 meters), and for injury crashes. For injury crashes, intersection angle and minor road posted speed are significant. Models of crashes at
signalized intersections by approach flows are also investigated, and other model forms are proposed for future consideration.